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Re: [Xen-devel] [Xen 4.12 bug] HVM/PVH boot confusion
- To: Wei Liu <wei.liu2@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 30 Jan 2019 12:30:44 +0000
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- Cc: Juergen Gross <JGross@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Xen-devel List <xen-devel@xxxxxxxxxxxxx>
- Delivery-date: Wed, 30 Jan 2019 12:30:54 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 30/01/2019 11:59, Wei Liu wrote:
> On Tue, Jan 29, 2019 at 07:49:51PM +0000, Andrew Cooper wrote:
>> Hello,
>>
>> Given the following vm.cfg file:
>>
>> name="vm"
>> type="hvm"
>>
>> vcpus=4
>> memory=1024
>>
>> firmware_override="/root/xen-syms"
>>
>> kernel="/boot/vmlinuz-4.4-xen"
>> ramdisk="/boot/initrd-4.4.0+10.img"
>>
>> cmdline="console=xen,pv dom0=pv --- earlyprintk=xen"
>>
> What is your use case here? If you want to use the shim, why don't you
> put pvshim=1 directly?
I'm testing a corner case.
>
>> Xen crashes with the following trace:
>>
>> (d15) (XEN) Xen BUG at pvh-boot.c:82
>> (d15) (XEN) ----[ Xen-4.12.0-rc x86_64 debug=y Not tainted ]----
>> (d15) (XEN) CPU: 0
>> (d15) (XEN) RIP: e008:[<ffff82d0804331f2>] pvh_init+0x27d/0x2fe
>> <snip>
>> (d15) (XEN) Xen call trace:
>> (d15) (XEN) [<ffff82d0804331f2>] pvh_init+0x27d/0x2fe
>> (d15) (XEN) [<ffff82d080429000>] __start_xen+0x14c/0x28f6
>> (d15) (XEN) [<ffff82d0802000f3>] __high_start+0x53/0x55
>> (d15) (XEN)
>> (d15) (XEN)
>> (d15) (XEN) ****************************************
>> (d15) (XEN) Panic on CPU 0:
>> (d15) (XEN) Xen BUG at pvh-boot.c:82
>> (d15) (XEN) ****************************************
>>
>> The problem is that Xen is started at its PVH entrypoint (contrary to
>> the instructions in the vm config file), and Xen unconditionally expects
>> RSDP to be passed.
>>
>> There are at least two bugs here.
>>
>> 1) RSDP was a late addition to the PVH boot protocol. Xen's PVH
>> entrypoint must not mandate its existence, because there are releases of
>> the domain builder which don't provide it.
> The inner Xen, in this case, can fall back to scanning low 1MB memory
> for RSDP.
Agreed.
>
>> 2) The HVM/PVH boot confusion. This think this is a still-outstanding
>> bug around the broken assumption that the hvmloader binary speaks the
>> PVH protocol without advertising itself appropriately (I really regret
>> not objecting to those patches before they went in). At the least, that
>> needs fixing by putting a proper ELF note in hvmloader, and the domain
>> builder needs to be updated to build all PVH-boot-ABI images consistently.
> Do you expect users to drop an arbitrary hvmloader into an arbitrary
> version of Xen and they continue to work?
This is hard to answer. HVMLoader specifically, perhaps not. After
all, it does link unstable libraries from tools/
That said, how we boot guests is an ABI, and this should be kept in mind
when making changes.
~Andrew
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